Various suspension systems have been used in vehicles. Depending upon the type of vehicle, the suspension system may include coil springs, leaf springs, oil or air-type shock absorbers or torsion rods.
The suspension system of one known vehicle includes a single torsion rod coupled to each wheel. Each of the torsion rods provides suspension for a particular wheel. When the vehicle has a full load, each of the torsion rods can reach a maximum absorption or twisting position. At this maximum absorption position, the torsion rods can provide no further suspension. Accordingly, one of disadvantages of this vehicle is that impacts can damage the vehicle if, for example, the operator places a full load on the vehicle and drives the vehicle over a bump or other irregularity in the driving surface.
In an attempt to overcome this disadvantage, another know vehicle described in U.S. Pat. No. 5,411,287 uses a suspension system having four torsion shafts positioned along a common axis within the rear axle of the vehicle. A pair of interconnected torsion shafts closest to each rear wheel provides suspension for that rear wheel. For each rear wheel, when the vehicle has no load or a relatively light load, only one of the torsion shafts provides suspension for that wheel. When the vehicle has a full load, this torsion shaft can reach a maximum or absorption or twisting position. At this position, this torsion shaft provides little or no suspension for such wheel. When this occurs, the other torsion shaft provides suspension for that wheel. Therefore, at any such point in time, only one torsion shaft has a relatively  significant role in providing suspension to a rear wheel of the vehicle. In addition, with the torsion shafts being aligned along a common axis, this suspension system limits the vertical travel of the wheels to a relatively small range, which, in turn, limits the suspension of the vehicle. For these reasons, when this vehicle is under a full load, the suspension can be exhausted. Accordingly, one of the disadvantages of this vehicle is that impacts can damage the vehicle if the operator places a full load on the vehicle and drives the vehicle over a bump or other irregularity in the driving surface.
Therefore, there is a need to overcome each of these disadvantages.